Background
With the rapid development of wireless communication devices, the amount of data traffic generated and required to be transmitted has increased exponentially. The rapid increase of data traffic has seriously affected the smooth transmission of the current wireless communication frequency band, so that a new wireless communication frequency band application needs to be developed to overcome the technical bottleneck. Millimeter wave communication works in a frequency band of 30-300GHz, and compared with a microwave frequency band used in traditional cellular communication, millimeter wave communication can provide wider communication bandwidth, so that the problem of spectrum blockage can be better solved, and the millimeter wave communication is regarded as a communication technology with wide application prospect. However, many technical challenges are still faced in implementing millimeter wave communication into practical applications. On one hand, compared with the traditional frequency band (2.4GHz), the millimeter wave communication carrier frequency is improved by more than ten times, under the condition, the atmospheric absorption and the rain attenuation are greatly improved, the signal penetration capacity is reduced, and the transmission capacity is weakened. In order to solve the above problems, it is a conventional practice to combine the millimeter wave communication technology with the massive MIMO technology tightly, and provide a sufficient beamforming gain under an advanced beamforming and combining mechanism. On the other hand, the hardware implementation of millimeter wave communication is also a difficult point that restricts the popularization and application of the millimeter wave communication. In a traditional all-digital beam forming architecture, signals transmitted or received by each antenna are processed by a series of radio frequency devices such as DAC/ADC. Since the power consumption and the manufacturing cost of these rf devices increase with the increase of the communication frequency and bandwidth, and the millimeter wave communication frequency and the signal bandwidth are both significantly increased compared to the conventional frequency band, it is difficult to manufacture and use a large number of rf devices with high precision due to the constraint of economic cost. In addition, in the millimeter wave communication system, a large-scale antenna array must be adopted to resist higher channel attenuation, and the extremely short wavelength of the millimeter wave signal can also package the large-scale antenna in a smaller size. Therefore, in the millimeter wave communication system, it is difficult to perform precise amplitude and phase control on signals on each antenna through processing of components in the radio frequency chain. The above problems are all technical and application bottlenecks that limit the wide application of the all-digital beam forming structure in the wireless communication field.
Recently, the analog/digital hybrid beamforming technology has been applied to millimeter wave communication in an attempt to save energy due to its economical features, which has the advantages of requiring a smaller number of radio frequency devices and being able to meet the user's usage requirements. The hybrid beam forming structure divides digital signal processing into two parts, namely digital beam forming is carried out on baseband signals by utilizing radio frequency chain components to ensure the realization of multiplexing or multi-user technology, and the front end of the antenna carries out analog beam forming modulation on the radio frequency signals through a large-scale phase shifter network to provide sufficient beam forming gain. It is worth proposing that analog beamforming only modulates the phase of the signal, without changing the signal amplitude. However, this structure also causes a significant power consumption because of the large number of phase shifters. In order to reduce the number of phase shifters, another partial connection structure is proposed, that is, the signal processed and output by each rf chain is only connected to a part of the antennas, so that the number of phase shifters required is effectively reduced, and thus the energy utilization rate is improved, but the beamforming gain is inevitably reduced. In addition, studies have shown that the power consumption and hardware complexity of the phase shifter network for analog beamforming is proportional to the phase shifter resolution, so the high-precision phase shifter network in such a partially connected hybrid architecture still causes huge energy consumption. In order to further reduce the hardware complexity and save the energy consumption, researchers have proposed a new structure, that is, each data stream passes through the radio frequency chain, and after frequency conversion, the switch dynamically selects which antenna to connect to. This architecture, while having very low hardware complexity, requires a small number of switches, and consumes very little power, the resulting beamforming gain is very limited.
Disclosure of Invention
In light of the above-identified problems, a hybrid beamforming structure for millimeter wave multiple antenna systems is provided. The invention mainly utilizes the phase over-sampler and a switch network to realize the analog beam forming, thereby realizing the maximization of hardware efficiency and simultaneously providing enough beam forming gain to achieve the ideal signal processing effect.
The technical means adopted by the invention are as follows:
a hybrid beamforming structure for use in a millimeter wave multiple antenna system, comprising:
a digital beamforming filter that processes the amplitude and phase of the signal at baseband;
obtaining a radio frequency chain of a radio frequency signal through digital-to-analog conversion and frequency conversion processing;
and an analog beamforming filter modulating the phase of the radio frequency signal to generate sufficient beamforming gain and transmitting to an antenna, the analog beamforming filter including a number of phase oversamplers equal to the number of radio frequency chains and a switching network;
the phase of the signal is modulated in different degrees through the phase over-sampler, then the signal is output in parallel, and the optimal modulation signal is selected by utilizing the switching network dynamic based on the criterion of maximizing the channel gain, so that the phase modulation of the signal is completed, and finally the signal is transmitted to each antenna.
Further, when the phase over sampler is a two-phase over sampler, the phase over sampler is realized by an inverter;
when the phase over-sampler is a four-phase or more phase sampler, phase delay is realized by a micro-strip delay line in a narrow-band communication environment; in a broadband communication environment, phase delay is implemented by an inductive-capacitive based broadband phase shifter.
Further, the resolution of the phase over sampler is N, and the signals of 0,2 pi/N, …,2(N-1) pi/N phase modulation are output in parallel.
Further, the output of the phase over-sampler is connected to a variable gain amplifier to provide gain compensation for different phases.
The invention also provides a signal transmission system applying the hybrid beam forming structure, which is characterized by comprising a transmitting end and a receiving end, wherein the transmitting end and the receiving end both adopt the hybrid beam forming structure, and the hybrid beam forming structures of the transmitting end and the receiving end are symmetrically arranged.
Compared with the prior art, the invention has the following advantages:
the method utilizes the more economical and energy-saving phase over-sampler and a switch network to realize analog beam forming, thereby maximizing the hardware efficiency and simultaneously providing enough beam forming gain to achieve the ideal signal processing effect. The structure has very simple circuit and small quantity of required phase over-samplers, and can perform phase modulation on signals to different degrees at the transmitter end through the phase over-samplers, then output the signals in parallel, dynamically select the optimal output through a switch network to complete the analog beam forming modulation on the signals, and finally transmit the signals to each antenna. The receiver end can also adopt the structure to complete similar processes.
By applying the technical scheme of the invention, the signal processing process is divided into a digital domain and an analog domain, thereby greatly reducing the number of radio frequency devices such as DAC/ADC and the like, reducing the hardware complexity and power consumption, solving the problems of complex hardware structure and insufficient beam forming gain of the existing transmission system, realizing the maximization of hardware efficiency and achieving the ideal signal processing effect.
Based on the reason, the invention can be widely popularized in the fields of millimeter wave communication, large-scale MIMO systems and the like.
Detailed Description
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict. The present invention will be described in detail below with reference to the embodiments with reference to the attached drawings.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The relative arrangement of the components and steps, the numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless specifically stated otherwise. Meanwhile, it should be understood that the sizes of the respective portions shown in the drawings are not drawn in an actual proportional relationship for the convenience of description. Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail but are intended to be part of the specification where appropriate. Any specific values in all examples shown and discussed herein are to be construed as exemplary only and not as limiting. Thus, other examples of the exemplary embodiments may have different values. It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof is not required in subsequent figures.
The invention provides a mixed beam forming structure applied to a millimeter wave multi-antenna system, which adopts an analog/digital mixed beam forming technology, namely, a signal processing process is divided into a digital domain and an analog domain, a small number of radio frequency devices are utilized to complete the digital beam forming process so as to ensure the realization of multiplexing or multi-user technology, the front end is connected with an antenna through a large-scale analog component to realize analog beam forming to compensate the attenuation loss of a channel and provide processing gain, a phase over-sampler and a switch network are adopted to realize the function of analog beam forming, the phase of a signal is modulated in different degrees through the phase over-sampler and then is output in parallel, and the optimal modulation signal is dynamically selected by the switch network so as to complete the phase modulation of the signal and finally transmitted to each antenna. The criterion for determining the best modulation signal is to maximize the channel gain, i.e. to select the modulation phase that best matches the channel matrix.
As shown in fig. 1, the structure of the present invention specifically includes: a digital beamforming filter that processes the amplitude and phase of the signal at baseband to provide multiple data stream transmission; obtaining a radio frequency chain of a radio frequency signal through digital-to-analog conversion and frequency conversion processing; and an analog beamforming filter modulating the phase of the radio frequency signal to generate sufficient beamforming gain and transmitting to an antenna, the analog beamforming filter including a number of phase oversamplers equal to the number of radio frequency chains and a switching network; the phase of the signal is modulated to different degrees through the phase over-sampler, the resolution of the phase over-sampler is N, the signal of 0,2 pi/N, …,2(N-1) pi/N phase modulation is output in parallel, the optimal modulation signal is dynamically selected by utilizing a switch network, and therefore the phase modulation of the signal is completed, and finally the signal is transmitted to each antenna.
Compared with the traditional numerical control phase shifter, the phase over-sampling device only needs a small number of phase over-sampling devices, the number of the phase over-sampling devices is equal to that of the radio frequency chains, namely, before each path of signal of the transmitting end is transmitted, the signal with different phases is output through the processing of each component in the radio frequency chains and the processing of the phase over-sampling devices after up-conversion, and finally, the signal is selectively connected to each antenna through the switch network.
The specific advantages of the system architecture based on the phase over sampler-switch network (POS-SW) are mainly shown in the following aspects: the number of the phase over-samplers is small, the implementation is simpler than that of the traditional numerical control phase shifter, and in addition, the power consumed by the switch is also small. For example, the binary phase over sampler can be implemented by a simple inverter, as shown in fig. 2(a), which is a preferred example of an implementation capable of implementing this function. Four-phase and more-phase oversamplers can be realized by phase delay means. For a narrow-band system, the phase delay can be implemented by using a microstrip delay line as shown in fig. 2(b), which is very easy to implement and has low energy consumption. But phase non-linearity problems (different frequency phase offsets) may occur. Therefore, for a broadband communication environment, a phase delay process can be realized by using a broadband phase shifter based on an inductance and a capacitance to improve the problem of phase nonlinearity. In addition, since the number of antennas connected to the output of the over sampler may vary, causing different circuit loads, a variable gain amplifier is often required at the output of each phase delayed signal to provide gain compensation for different phases. Finally, it is worth proposing that experiments show that usually one switch circuit consumes 5mW of energy, the microstrip delay line consumes no power, one phase shifter generally consumes 45-106mW, and the number of phase shifters required by the conventional phase shifter network is very large, while the number of phase over-samplers in the structure proposed by the present invention is much smaller, so that the total energy consumption is greatly reduced, and the hardware complexity is also significantly reduced.
The invention also provides a signal transmission system applying the mixed beam forming structure, which comprises a transmitting end and a receiving end, wherein the transmitting end and the receiving end both adopt the mixed beam forming structure, and the mixed beam forming structures of the transmitting end and the receiving end are symmetrically arranged.
Example 1
The technical solution of the present invention is further explained below by taking the design of hybrid beamforming based on the phase over-sampler-switch network structure as an example.
Taking a point-to-point millimeter wave MIMO communication system as an example, by using the hybrid beamforming structure of the present invention, assume that the transmitter has N
tRoot antenna and
a radio frequency chain, transmission N
sA data stream, similarly provided that the receiver has N
rRoot antenna and
a radio frequency chain. The signal transmitted by the transmitter passes through the base band digital beam former
By the treatment of
D/A conversion and frequency conversion of the RF chain are processed into RF signals, which are then processed by an analog beam former
The processing of (1) transmits the signal to the antenna. Similarly, at the receiver end, a combining matrix W is first merged by an analog
RFThe signal is received with similar constraints as the analog beamforming matrix, i.e. constant amplitude and quantized phase with resolution N, down-converted and then combined with a digital combining matrix W
BBThe signal is estimated. Let H be the channel matrix, s be the transmitted signal matrix, P be the transmitted power, and n be the received complex Gaussian noise vector. The received signal can be represented in the form:
the design objective equation of the hybrid beamforming and combining matrix based on the phase over-sampler-switch network structure maximizes the spectral efficiency. The design difficulty mainly comes from the low resolution phase limit of the analog beamforming matrix. The complexity of the optimal exhaustive search algorithm increases exponentially with the increase of the number of antennas, so that in practical application, the exhaustive search cannot be adopted to design a simulation beam forming or combining matrix, and a more efficient and simple algorithm needs to be designed to realize the simulation beam forming. The invention adopts a design algorithm of a novel combined analog beam forming and merging matrix with low complexity (reference document: Wang Z, Li M, Liu Q et al.
Specifically, for the case where N > 2, an iterative phase matching algorithm is adopted, and a flowchart is shown in fig. 3, which includes the following steps:
s1: carrying out SVD on the channel matrix;
s2: iteratively updating the phase of each analog beamforming pair based on the results of the SVD decomposition;
s3: multiplying the designed analog beam forming matrix into a channel matrix to obtain an effective baseband channel matrix;
s4: and carrying out SVD on the effective baseband channel matrix, and enabling the digital beam forming matrix to be a left singular matrix and a right singular matrix of the effective baseband channel matrix respectively.
For the case of a binary phase oversampler (i.e., N-2), a method of rank 1 approximation is used, and the flow chart is shown in fig. 4, with the following steps:
s1: carrying out SVD on the channel matrix;
s2: designing a codebook by using a rank 1 approximation algorithm based on the result of the SVD;
s3: selecting an optimal analog beamforming pair from a codebook to maximize channel gain;
s4: multiplying the designed analog beam forming matrix into a channel matrix to obtain an effective baseband channel matrix;
s5: and carrying out SVD on the effective baseband channel matrix, and enabling the digital beam forming matrix to be a left singular matrix and a right singular matrix of the effective baseband channel matrix respectively.
As shown in fig. 5, based on the proposed architecture of the present invention, when N is 8, the system performance has already approached the full digital beamforming structure. Finally, as can be seen from the simulation result of fig. 6, the architecture proposed by the present invention has a very significant advantage in energy efficiency performance.
Example 2
This embodiment is a channel estimation embodiment based on a phase over sampler-switch network structure.
The narrow-band millimeter wave channel often adopts a cluster model, namely the channel matrix H is considered to be composed of NCLFormed by co-operation of scattering clusters, each cluster providing NrayA strip propagation path. The physical channel model can then be expressed in the form:
in the formula betai,lIs the complex gain of the i-th propagation path in the i-th scattering cluster, aMS(φi,L) And aBS(ai,l) The antenna array response vectors phi of the receiving end and the transmitting end aiming at the ith propagation path in the ith scattering cluster respectivelyi,lAnd alphai,lFor the corresponding arrival azimuth and departure azimuth. The channel matrix may also be represented as follows:
in the formula
And
the antenna array response vector, H, comprising the azimuth angles
bIs a diagonal matrix, with values on the diagonal corresponding to the corresponding path gain β
i,l。
The design of the hybrid beamforming and combining matrix requires information of the channel matrix H, but millimeter wave channel information is not readily available. The reason for this is that the received signal is first filtered by an analog beamforming matrix, and the baseband cannot directly estimate the value of each element of the channel matrix H. As can be seen from the description of the millimeter wave channel model, the millimeter wave channel is sparse, and based on the characteristics, methods such as compressed sensing and the like can be used for millimeter wave channel estimation. Based on the phase over sampler-switch network structure, the elements of the training sequence should be discrete values, so that, based on the POS-SW structure, as shown in fig. 7, the present invention adopts a channel estimation algorithm based on compressed sensing, and the steps are as follows:
s1: the transmitter transmits uniformly distributed random sequences in each time slot;
s2: the receiver stores the received signal of each time slot;
s3: the receiver end carries out channel estimation by utilizing an OMP algorithm;
s4: the receiver end feeds back the estimated value to the transmitter.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.